The co-pending, commonly-assigned application Ser. No. 08/623,822 entitled "A METHOD AND APPARATUS FOR DETERMINING THE CENTER AND ORIENTATION OF A WAFER-LIKE OBJECT," filed Mar. 29, 1996, by Tac Huynh, now U.S. Pat. No. 5,822,215 discloses a fixedly positioned line or sheet source of optical energy in an apparatus for determining the center of a generally circular semiconductor wafer relative to an axis of a spindle for turning the wafer through one revolution. The source continuously projects onto the semiconductor wafer top face a line image that extends along a radial line between the spindle axis and the wafer periphery. The optical energy extends as a radial line beyond the wafer image for all reasonable positions of the wafer on a platform rotated by the spindle.
An optical detector fixedly positioned below the wafer includes a charge coupled device (CCD) linear array aligned with the line of optical energy projected toward the wafer. Hence, the linear CCD array extends from the spindle radially outward beyond the wafer periphery. As the spindle rotates the wafer, the amount of optical energy from the source incident on the CCD detector is directly proportional to the complement of the length of a line between the spindle axis of the wafer and the wafer periphery.
A computer coupled to an output of the CCD array responds to indications of the line length complement to calculate the displacement of the wafer center from the spindle rotation axis. The computer also determines the angle of wafer orientation relative to a reference angle by detecting the location of an abrupt change in curvature on the generally circular periphery of the wafer. The abrupt change can be detected since the wafer has one or more flats or a notch. The abrupt change is detected by differentiating the CCD output indicative of line length complement.
After the separation between the wafer center and spindle axis and the relative orientation of the abrupt change have been determined, a wafer aligner translates and rotates the wafer so the wafer center is on the spindle axis and the abrupt change has a predetermined orientation relative to the reference angle. The wafer, after being so positioned, is moved from the aligner to a processor of a wafer processing system.
We have realized that a problem with the prior art device is that a DC drive motor for the spindle is subject to turning at differing speeds. Consequently, output signals of the CCD array are not always exactly proportional to an indication of the length of the lines between the spindle axis and the wafer periphery and inaccuracies occur in the indications of the spindle and wafer rotation angles. These inaccuracies tend to prevent the wafer center and flat or notch positions from being accurately determined, whereby the wafer center is not accurately positioned on the spindle axis. Consequently, the wafers are subject to being incorrectly positioned in the processing chambers. Therefore, the processed wafers are subject to not having the full complement of integrated circuits produced thereon.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for determining the position of a generally circular workpiece center relative to a rotation axis of the workpiece.
A further object of the invention is to provide a new and improved method of and apparatus for determining the position of the center of a generally circular workpiece relative to a turning axis of the workpiece using a line source of optical radiation.
Another object of the present invention is to provide an apparatus for and method of overcoming inaccuracies in determining the position of a wafer center relative to an axis of a spindle which turns the wafer in an apparatus to determine the relative position of the wafer center and the spindle axis, wherein tendencies of a drive motor for the spindle to turn the wafer at differing speeds is overcome.